This invention relates to the preparation of asphalt polymer compositions through the use of a multi-component crosslinking system incorporating a thiopolymer and at least one other crosslinking component.
The use of bitumen (asphalt) compositions in preparing aggregate compositions (bitumen +rock) useful as road paving material is complicated by at least three factors, each of which imposes a serious impediment to providing an acceptable product. First, the bitumen compositions must meet certain performance criteria or specifications in order to be considered useful for road paving. For example, to ensure acceptable performance, state and federal agencies issue specifications for various bitumen applications including specifications for use as road pavement. Performance standards and properties relating to asphalt cements are set forth in various standards of the American Society for Testing and Materials (ASTM) and the American Associate of State Highway and Transportation Officials (AASHTO). Current Federal Highway Administration specifications designate a bitumen (asphalt) product, for example, AC-20R (xe2x80x9cRxe2x80x9d meaning rubber modified), as meeting defined parameters relating to properties such as viscosity, toughness, tenacity and ductility. Each of these parameters define an important feature of the bitumen composition and compositions failing to meet one or more of these parameters may well render that composition unacceptable for use as road pavement material. Table 1 sets forth the properties of various grades of asphalt cement (AC) as determined in accordance with AASHTO M-226 (Specification for Viscosity-Graded Asphalt Cement).
Conventional bitumen compositions frequently cannot meet all of the requirements of a particular specification simultaneously and, if these specifications are not met, damage to the resulting road can occur, including permanent deformation, thermally induced cracking and flexural fatigue. This damage greatly reduces the effective life of paved roads.
In this regard, it has long been recognized that the properties of conventional bitumen compositions can be modified by the addition of other substances, such as polymers. A wide variety of polymers have been used as additives in bitumen compositions. For example, copolymers derived from styrene and conjugated dienes, such as butadiene or isoprene, are particularly useful, since these copolymers have good solubility in bitumen compositions and the resulting modified-bitumen compositions have good Theological properties.
It is also known that the stability of polymer-bitumen compositions can be increased by the addition of crosslinking agents such as sulfur, frequently in the form of elemental sulfur. It is believed that the sulfur chemically couples the polymer and the bitumen through sulfide and/or polysulfide bonds. The addition of extraneous sulfur is required to produce the improved stability, even though natural bitumens naturally contain varying amounts of native sulfur.
Thus, U.S. Pat. No. 4,145,322, issued Mar. 20, 1979 to Maldonado et al., discloses a process for preparing a bitumen-polymer composition consisting of mixing a bitumen, at 266xc2x0-446xc2x0 F. (130xc2x0-230xc2x0 C.), with 2 to 20% by weight of a block copolymer, having an average molecular weight between 30,000 and 300,000, with the theoretical formula Sxxe2x80x94By, in which S corresponds to styrene structure groups and B corresponds to conjugated diene structure groups, and x and y are integers. The resulting mixture is stirred for at least two hours, and then 0.1 to 3% by weight of sulfur relative to the bitumen is added and the mixture agitated for at least 20 minutes. The preferred quantity of added sulfur cited in this patent is 0.1 to 1.5% by weight with respect to the bitumen. The resulting bitumen-polymer composition is used for road coating, industrial coating, or other industrial applications.
U.S. Pat. No. 4,130,516, issued Dec. 19, 1978 to Gagle et al., discloses an asphalt (bitumen) polymer composition obtained by hot-blending asphalt with 3 to 7% by weight of elemental sulfur and 0.5 to 1.5% by weight of a natural or synthetic rubber, preferably a linear, random butadiene/styrene copolymer. U.S. Pat. No. 3,803,066, issued Apr. 9, 1974 to Petrossi, also discloses a process for preparing a rubber-modified bitumen by blending rubber, either natural or synthetic, such as styrene/butadiene rubber, with bitumen at 293xc2x0-365xc2x0F. (145xc2x0-185xc2x0 C.), in an amount up to 10% by weight based on the bitumen, then adjusting the temperature to 257xc2x0-320xc2x0 F. (125xc2x0-160xc2x0C.), and intimately blending into the mix an amount of sulfur such that the weight ratio of sulfur to rubber is between 0.3 and 0.9. A catalytic quantity of a free-radical vulcanization-accelerator is then added to effect vulcanization. This patent recites the critical nature of the sulfur to rubber ratio, and teaches that weight ratios of sulfur to rubber of less than 0.3 gives modified bitumen of inferior quality.
Although polymer-modified bitumen compositions are known, these previously described compositions are not necessarily useful for road paving applications. For example, mixing NorthWest paving asphalt having an initial viscosity of 682 poise at 140xc2x0 F. (60xc2x0 C.) with 3.6 weight percent Kraton(copyright)-4141, a commercially available styrene-butadiene tri-block copolymer which contains 29 weight percent plasticizer oil, and 0.25% sulfur gives a modified-asphalt composition with a viscosity of 15,000 poise at 140xc2x0 F. (60xc2x0 C.). This viscosity, however, greatly exceeds the acceptable viscosity range set by the widely used AC-20R specification for paving asphalt. This specification, issued by the Federal Highway Administration, requires bitumen compositions to have a viscosity in the range of 1600-2400 poise at 140xc2x0 F. (60xc2x0 C.). Thus, the modified bitumen compositions produced by the procedures of U.S. Pat. No. 4,145,322 using Kraton(copyright)-4141 would be unacceptable for use in road paving under the AC-20R specification.
A second factor complicating the use of bitumen compositions involves the viscosity stability of such compositions under storage conditions. In this regard, bitumen compositions are frequently stored for up to 7 days or more before being used and, in some cases, the viscosity of the composition can increase so much that the bitumen composition is unusable for its intended purpose. On the other hand, a storage stable bitumen composition would provide for only minimal viscosity increases and, accordingly, after storage it can still be employed for its intended purpose.
A third factor complicating the use of bitumen compositions involves the use of volatile solvents in such compositions. Specifically, while such solvents have been heretofore proposed as a means to fluidize bitumen-polymer compositions containing relatively small amounts of sulfur which compositions are designed as coatings (Maldonado et al., U.S. Pat. No. 4,242,246), environmental concerns restrict the use of volatile solvents in such compositions. Moreover, the use of large amounts of volatile solvents in bitumen compositions may lower the viscosity of the resulting composition so that it no longer meets viscosity specifications designated for road paving applications. In addition to the volatile components, reduction of other emissions during asphalt applications becomes a target. For example, it is desirable to reduce the amount of sulfur compounds that are emitted during asphalt applications.
Asphaltic concrete, typically including asphalt and aggregate, asphalt compositions for resurfacing asphaltic concrete, and similar asphalt compositions must exhibit a certain number of specific mechanical properties to enable their use in various fields of application, especially when the asphalts are used as binders for superficial coats (road surfacing), as asphalt emulsions, or in industrial applications. (The term xe2x80x9casphaltxe2x80x9d is used herein interchangeably with xe2x80x9cbitumen.xe2x80x9d Asphaltic concrete is asphalt used as a binder with appropriate aggregate added, typically for use in roadways.) The use of asphalt or asphalt emulsion binders either in maintenance facings as a surface coat or as a very thin bituminous mix, or as a thicker structural layer of bituminous mix in asphaltic concrete, is enhanced if these binders possess the requisite properties such as desirable levels of elasticity and plasticity.
Previously, various polymers have been added to asphalts to improve physical and mechanical performance properties. Polymer-modified asphalts are routinely used in the road construction/maintenance and roofing industries. Conventional asphalts often do not retain sufficient elasticity in use and, also, exhibit a plasticity range that is too narrow for use in many modem applications such as road construction. It is known that the characteristics of road asphalts and the like can be greatly improved by incorporating into them an elastomeric-type polymer which may be one such as butyl, polybutadiene, polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer, polyacrylate, polymethacrylate, polychloroprene, polynorbornene, ethylene/propylene/diene (EPDM) terpolymer and advantageously a random or block copolymer of styrene and a conjugated diene. The modified asphalts thus obtained commonly are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. Modified asphalts and asphalt emulsions typically are produced utilizing styrene/butadiene based polymers, and typically have raised softening point, increased viscoelasticity, enhanced force under strain, enhanced strain recovery, and improved low temperature strain characteristics.
The bituminous binders, even of the bitumen/polymer type, which are employed at the present time in road applications often do not have the optimum characteristics at sufficiently low polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration.
Current practice is to add the desired level of a single polymer, sometimes along with a reactant that promotes cross-linking of the polymer molecules until the desired asphalt properties are met. This reactant typically is sulfur in a form suitable for reacting. Such current processes are discussed in various patents such as U.S. Pat. Nos. 4,145,322 (Maldonado); 5,371,121 (Bellamy); and 5,382,612 (Chauerat), all of which are hereby incorporated by reference.
However, the cost of the polymer adds significantly to the overall cost of the resulting asphalt/polymer mix. Thus, cost factors weigh in the ability to meet the above criteria for various asphalt mixes. In addition, at increasing levels of polymer concentration, the working viscosity of the asphalt mix becomes excessively great and separation of the asphalt and polymer may occur.
One result of the high viscosities experienced at increased polymer concentrations is that it makes emulsification of the asphalt difficult. As is known in the art and used herein, emulsification of asphalt refers to forming an emulsion of asphalt and water. Asphalt emulsions are desirable in many applications because the emulsion may be applied at lower temperatures than hot-mix asphalts since the water acts as a carrier for the asphalt particles.
For example, hot-mix asphalts, mixes of asphalt, aggregate, and a single polymer, commonly are applied at a temperature within the range of 350xc2x0 F. to 450xc2x0 F. (177xc2x0 C. to 232xc2x0 C.) to achieve the requisite plasticity for application. In comparison, an asphalt emulsion typically may be applied at temperatures within the range of 130xc2x0 F. to 170xc2x0 F. (54xc2x0 C. to 77xc2x0 C.) to achieve the same working characteristics. Emulsified asphalt products are generally used to reduce the release of environmentally-harmful volatile organic compounds often associated with asphalts diluted with light carrier solvents such as diesel fuel, naphtha, and the like. Emulsification basically requires that the asphalt and any desired performance-enhancing additives be combined with an emulsifying agent in an emulsification mill along with about 20 to 40 percent by weight of water. However, high polymer loading in asphalt produces high viscosities and melting points, making emulsification of the polymer-asphalt composition difficult.
The bitumen/polymer compositions are prepared in practice at polymer contents which range from about 3% to 6% by weight of bitumen depending on the nature and the molecular weight of the polymer and the quality of the bitumen. Gelling of the bitumen/polymer composition, which is observed fairly frequently during the preparation of the composition or while it is stored, occurs as soon as the polymer content of this composition exceeds the above-mentioned threshold. It is thus difficult, in practice, to produce non-gellable bitumen/polymer compositions with a high polymer content, which are more economical to prepare and to transport than bitumen/polymer compositions with a lower polymer content.
U.S. Pat. No. 6,180,697, xe2x80x9cMethod for Preparation of Stable Bitumen Polymer Composition,xe2x80x9d Kevin Kelly and James R. Butler, discloses methods for the preparation of asphalt and thermoplastic elastomer compositions. The procedure in U.S. Pat. No. 6,180,697 involves the addition of a thermoplastic elastomer in concert with crosslinking formulations which include elemental sulfur, zinc-2-mercaptobenzothiazole, zinc oxide, and dithiodimorpholine as a replacement for an equivalent amount of elemental sulfur. Asphalt compositions made by this method comply with the standards for asphalt cement Grade AC-20.
In accordance with the present invention there is provided a process for the preparation of an asphalt and polymer composition in which a minor amount of polymer component is subjected to a crosslinking. In carrying out the invention, an asphalt base material is heated in a mixing chamber at a temperature sufficient to melt the asphalt and allow stirring of the asphalt within the chamber. A thermoplastic elastomer is added to the chamber in an amount not greater than 12 wt. % based upon the composite of the elastomer and the asphalt. While the asphalt elastomer formulation is stirred, a crosslinking agent is added in an amount effective to crosslink the thermoplastic elastomer. The crosslinking agent formulation comprises at least two components. One component is a thiopolymer incorporating organic sulfur prepared by the reaction of 6-tertiary butyl cresol and sulfur dichloride which reacts to produce a reaction product comprising 4,4-thiobis(6-tertiary butyl m cresol) and the thiopolymer, followed by subsequent separation of the 4,4-thiobis(6-tertiary-butyl-m-cresol) from the thiopolymer product. The second crosslinking component comprises a source of sulfur to crosslink the thermoplastic elastomer.
In a preferred embodiment of the invention, the second crosslinking component is selected from the group consisting of elemental sulfur, polythiomorpholine, preferably dithiodimorpholine, zinc-2-mercaptothiazole and mixtures thereof. In yet a further aspect of the invention, the secondary crosslinking agent comprises elemental sulfur. In a further aspect of the invention the secondary crosslinking agent is selected from the group consisting of dithiodimorpholine, zinc-2-mercaptobenzothiazole, or a mixture of zinc-2-mercaptothiozole and dithiodimorpholine. In yet a further aspect of the invention, the secondary crosslinking agent comprises a mixture of elemental sulfur and at least one zinc compound selected from the group consisting of zinc oxide and zinc-2-mercaptobenzothiazole.